package dujiu.luogu.circulation;

import java.util.Scanner;

public class P1720 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double n = scanner.nextDouble();
        scanner.close();
        double a1 = (1+Math.sqrt(5))/2.0;
        double a2 = (1-Math.sqrt(5)) /2.0;
        if (n == 0){
            System.out.printf("%.2f",n);
        }else if (n == 1){
            System.out.printf("%.2f",n);
        }else {
            for (int i = 1; i < n; i++) {
                a1 *= (1+Math.sqrt(5))/2.0;
                a2 *= (1-Math.sqrt(5))/2.0;
            }
            double a = (a1  - a2)/Math.sqrt(5);
            System.out.printf("%.2f",a);
        }

    }
}
